Subterranean fluid containment tank

ABSTRACT

A modular fluid storage tank, intended for underground storage of water, utilizing alternating circular and octagonal ribs in the main body and curved, ribbed, end caps. Interconnection of the tanks uses rigid pipe or semi-flexible hose. The coupling between the pipe and the tank incorporates a fitting, integrally molded into the tank, which extends into the interior of the tank, and a flexible sleeve which fits over, and is clamped to, the fitting and the pipe. The coupling is protected from the environment and is accessible from within the tank after installation for inspection and repair without the need to excavate the tank. The tank preferably has approximately equal length and diameter for improved strength. It is relatively small and lightweight, suitable for transport to and installation at a remote location without heavy equipment.

FIELD OF THE INVENTION

The present invention relates to the subterranean storage of fluids andin particular to the underground storage of water or for septic tanks ina system of modular tanks.

BACKGROUND OF THE INVENTION

The need for storage of water is well known, especially in the westwhere many regions are arid or semi-arid. Water may be stored for avariety of reasons: potable water for household use; water forlivestock; and fire cisterns are some of the more common applications.While open, above ground storage is common for livestock, enclosedstorage is preferred for potable water in order to avoid contamination.Where enclosed tanks are used, underground tanks are often preferred. Byburying the tank, the tank itself is supported, the tank is protected,and the contents of the tank are insulated from temperature changes.Underground tanks are also commonly used as septic tanks where municipalsewer service is not available.

Various materials have been used to construct underground storage tanks.Steel and concrete have been in use for decades. Fiberglass is a newermaterial rapidly gaining popularity especially in petroleum storage.Steel tanks are prone to rusting, especially where they are exposed toground water. Concrete tanks do not rust, but are semi-porous and willdeteriorate with time. Fiberglass has good resistance to corrosion, butis relatively brittle, requiring careful handling, especially duringinstallation. A sharp blow or inadvertent contact with the installationequipment can easily damage a fiberglass tank.

Both steel and concrete tanks are relatively heavy. This typicallyresults in the tanks being constructed relatively near the point ofinstallation to reduce transportation difficulties and expense. Thisweight also effectively limits the maximum size of tank which can beconstructed of these materials. Fiberglass is a much lighter materialand can be used to fabricate a tank which is relatively rigid for itssize. This enables the construction of relatively large, light weighttanks which are efficient to build and transport. However, there must besufficient room for the necessary equipment, such as a crane, at the jobsite to off load and install the tank. Additionally, construction offiberglass tanks is a labor intensive process which makes themrelatively expensive.

Polyethylene has several characteristics which make it particularlysuitable for potable water storage. It is typically formed into tanksusing a rotary molding process which results in a one piece, seamlesstank. This limits opportunities for bacterial growth and ground waterinfiltration. The material is impact resistant, flexing rather thancracking like fiberglass, and is highly corrosion resistant. However,this flexibility results in a vessel which is structurally weaker,limiting the size of the tank which can be cost effectively constructed.Large radiuses and flat surfaces are prone to buckling and collapse whenthe surrounding ground shifts or freezes, or from external pressure dueto ground water.

Where access to the site is restricted, none of the above approaches isideal. This is a common situation in mountain communities where theterrain limits access. Roads are often narrow and sharply curved.Surfaces may be rough and unimproved, with dirt or gravel being common.However, houses and ranches in these regions are often the ones most inneed of water storage because municipal water is unavailable, naturalwater supplies may not exist, and wells are difficult and expensive todrill and may provide only a minimal flow. In such a situation, a firecistern may be needed for safety and is often a prerequisite toobtaining fire insurance.

Compounding the problem is the fact that the required storage capacityoften exceeds the capacity of any single tank which can be efficientlymoved to the site and installed. It thus becomes necessary tointerconnect several tanks to form a storage system of sufficientcapacity. Several designs for such systems are known. Most common aretanks which couple together with mating pipe flanges. Where the terrainis uneven, achieving the required alignment to allow insertion of all ofthe bolts which join the flange can be very difficult. If it isachieved, shifting of the tanks due to settling can result insignificant stresses in the tanks. The strict alignment requirements canincrease the cost of the excavation as it must be matched to the tank,there being little adaptability in the tank. The level of skill requiredto properly install and join these types of tanks is often not availablein remote areas, requiring bringing in skilled labor for theinstallation. Further, supplies or materials may be required which arenot commonly available. If additional items are needed, the project maybe delayed while they are brought in.

Where the tank material is polyethylene, manufacturing characteristicsof the material introduce additional problems. Uneven cooling of themold and relative time of release of various portions of the tank fromthe mold cavity cause warping and distortion in the tank which can varyfrom one unit to another. While not significant to the performance ofthe tank, these variations can easily be large enough to make itdifficult to align all of the bolts for an interconnecting flange, andnearly impossible to align multiple connections on the face of a tank.

One solution to the alignment problem is to interconnect the tanks withflexible hose or tubing. This allows for misalignment between the tanksand eliminates the flange mating problem. However, the joints used forthis type of connection are prone to leaking and require periodicinspection and repair, which typically require at least partialexcavation. Additionally, the hardware components of the joint areexternal to the tank and are exposed to the environment and backfillmaterial both during and after installation.

There is a need for a relatively small, lightweight, modular tank whichcan be transported to the installation site over narrow, unimprovedroads using light to medium duty equipment. At the site, it should bepossible to easily unload and quickly connect several tanks together toprovide a larger storage system. Ideally, the system should providevarious configurations which are adaptable to the terrain or needs ofthe user. Alignment between the various modules must not be critical. Itshould be possible to install and connect the modules using unskilledlabor and materials which are commonly available in rural areas.Ideally, it should be possible to periodically inspect, and if necessaryrepair, the inter-tank connections after installation of the systemwithout excavation.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus for the storage ofliquids, primarily water, in an underground tank. The tank is adaptedfor use as a module in a larger storage system comprised of multipletanks.

According to the invention there is provided a tank with a ribbed mainbody and curved end caps and at least one fitting which extends inwardinto the cavity of the tank. The ribs alternate between outward circularribs, and inward octagonal ribs. Where the tanks will be positionedimmediately adjacent to each other, the tank can be manufacture with aflat panel on each end and the fitting positioned within the panel. Thefitting is initially formed as a fluid tight, integrally molded cup andthe end of the cup is cut out when needed to connect to a pipe.

According to an aspect of the invention, the shape of the tank is thatof a modified sphere, with the diameter and length of the tank beingsubstantially the same.

According to another aspect of the invention, a lengthwise rib may beadded to further stiffen the structure or the rib may encircle the tankalong a lengthwise circumference.

Further in accordance with the invention, one or more alignment cups maybe formed in the ends which accept short pipe segments for aligningadjacent tanks.

Still further in accordance with the invention, one or more spreadertubes may be positioned within the tank, pushing outward on the ends forincreased strength.

Yet further in accordance with the invention, the tanks may beconfigured into a system of tanks, interconnected by pipes, the pipescoupling to the tanks by means of a flexible sleeve and band clamps. Thesleeve and clamps are positioned within cavity of the tank. The sleevemay be of various forms or materials as necessary to provide therequired amount of flexibility in the joint. If desired, the pipe mayhave a smaller diameter than the fitting, increasing the tolerance forangular deflection.

The advantages of such an apparatus are a tank which is relatively smalland lightweight, and easily transported to remote sites. There, thetanks can be combined into a storage system of almost unlimitedcapacity. The tanks may be laid out in any configuration or orientationrequired to adapt to the local terrain. Interconnection of the tanks isachieved with simple readily available materials and can be performed byrelatively unskilled labor. The joints are within the tanks, protectedfrom the environment and readily accessible after installation forinspection or repair without excavation.

The above and other features and advantages of the present inventionwill become more clear from the detailed description of a specificillustrative embodiment thereof, presented below in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a perspective view of the tank.

FIG. 2 provides a top view of the tank

FIG. 3 provides an end view of tank

FIG. 4 provides a side view of tank

FIG. 5 is a cross section through the diameter of the tank illustratingthe rib structure.

FIG. 6 is a cross section through the length of the tank.

FIG. 7 provides a detailed view of the coupling between two tanks, alongthe same line as the cross section of FIG. 6.

FIG. 8 is a cross section through the coupling illustrating the standardflared sleeve.

FIG. 9 is a cross section through the coupling illustrating thealternative humped sleeve.

FIG. 10 is a cross section through the coupling illustrating thealternative S-curve sleeve.

FIG. 11 illustrates one possible configuration of multiple tanks as astorage system.

FIG. 12 illustrates a second possible configuration of multiple tanks asa storage system.

FIG. 13 is a cross section the length of an alternative embodiment ofthe tank utilizing spreader tubes.

FIG. 14 is a detail view illustrating the installation of a spreadertube over an alignment cup.

DETAILED DESCRIPTION OF THE INVENTION

The following discussion focuses on the preferred embodiment of theinvention, in which an underground modular storage tank for potablewater is presented. However, as will be recognized by those skilled inthe art, the disclosed apparatus is applicable to a wide variety ofsituations in which modular storage of fluids is desired. The preferredform is directly applicable to septic systems. With appropriate changesto the materials, the designs and techniques presented are alsoapplicable to petroleum and chemical storage. While the preferredinstallation method is to bury the tank, it may also be installedpartially exposed.

The following is a brief glossary of terms used herein. The supplieddefinitions are applicable throughout this specification and the claimsunless the term is clearly used in another manner.

Alignment cup—one or more relatively small depressions molded into theends of the tank used primarily for aligning adjacent tanks. Also usableas fluid connections if desired.

Band clamps—generally any type of compressive damp usable around thecircumference of a generally round object to contract around that objectproviding an inward force. One form of these, in relatively smalldiameters, are often referred to as hose clamps.

Connection cup—relatively large fitting molded into the ends of the tankused primarily for providing a fluid connection to the tank. Also usablefor alignment if desired.

Cup—in the present invention any of the various cup-shaped fittingsmolded into the end of the tank for use as a fluid connection, foralignment purposes, or both.

Cuff—the projecting edges of a connection cup to which the sleeve isclamped.

Pipe—herein, generally any elongated, hollow member capable oftransmitting fluids. Unless specifically restricted by context, thisterm is intended to include both pipe and tubing whether rigid orflexible and of any material.

Sleeve—generally any flexible elastomeric coupling for joining twosections of pipe or tubing. May also be referred to in the trade as aboot.

Preferred Embodiment

The disclosed invention is described below with reference to theaccompanying figures in which like reference numbers designate likeparts.

FIG. 1 provides an external isometric view of the inventive tank showingthe relative positioning of the major components and an overview of theentire structure. The individual elements will be discussed below withreference to the other figures which provide more detailed views.

With reference to FIG. 2, it can be seen that the overall structure ofthe tank is that of a modified sphere. While roughly cylindrical inshape, the ends, 100 and 102, are substantially curved and the diameter,D, is approximately equal to the length, L. The curve of the ends isinterrupted only by the end plates, 104 and 106, which are made flat toenable a flush interface between two adjacent tanks. Whereinterconnection of tanks is not anticipated, or where suchinterconnection will not be flush, the ends may be manufactured with acontinuous curve.

The modified spherical form has been found to be stronger than aconventional cylinder both in field tests and in vacuum tests where thetank is evacuated to various levels. Such vacuum tests were performedwith the tank above ground and exposed to normal atmospheric pressure.Preferably the length and diameter will be within 5% of each otheralthough differences of 15% have exhibited acceptable performance.Additional crush resistance is gained when the flat portions of the endsare braced with spreader tubes as described below.

The access opening, 108, provides ingress and egress for the tank. Aswith all of the openings, it is molded into the tank as a sealed surfacewith the opening appearing as a depressed area. If needed, this area iscut out, forming the opening. Where a single tank is to be used,eliminating the need to access the inside of the tank, the opening mayremain sealed. In the preferred embodiment, this area is also used toplace the vent which extends into the tank during the molding process.

With reference to FIGS. 3 & 4, the details of the external structure ofthe inventive tank can be seen. The main body of the tank is formed fromalternating outwardly, 112, and inwardly, 114, projecting ribs. Theseare discussed in more detail below. In a similar manner the ends of thetank comprise alternating ribs, 116, and troughs, 118. In the preferredembodiment, both ribs and troughs are formed from a series of flatplates. Alternatively, each of these could be formed as a continuouscurved surface. Centered on each end are end plates, 104 and 106, madeup of two sections, 104A, 104B and 106A, 106B, containing cups, 120 and122. The mold separation line, 110, has been enlarged and adapted toform a structural component of the tank. Of triangular cross section,see FIG. 5, this line forms an equatorial rib extending around thecircumference of the tank at the midpoint. This rib significantlystrengthens the tank, resisting expansion and contraction along thelength of the tank. Without this member, the ribs would freely accordionallowing the tank to expand and contract. An alternative embodiment ofthe tank includes a similar rib at the top and bottom of the tankextending along the centerline from end to end (interrupted by theaccess opening on the top). If desired, additional ribs could be added,spaced around the circumference of the tank, to further increase therigidity. However, it has been found that some ability to expand andcontract is desirable. An additional feature of the bottom rib would beto allow flow of water between the main ribs, preventing trapped water.

With reference to FIG. 5, the structure of the ribs can be seen in moredetail. The outwardly extending ribs, 112, are of conventional circularcross section. The inwardly extending ribs, 114, are of an octagonalcross section forming an eight sided structurally balanced matrix. Thiscombination of circular and octagonal ribs has been found to providestructural advantages as compared to using only circular ribs. Othernumbers of flat segments could be used, such as a hexagon. Thetriangular cross section of the side rib, 110, can be clearly seen. Thealternating pattern of the inward and outward ribs can be seen in FIG.6.

The cross section of FIG. 6 illustrates the positioning of the variouspotential openings provided for in the tank design. Connection cups,120, are intended for use as the primary fluid passage openings providedin the tank. There are four total, at the upper and lower limits of eachend of the tank. When the tank is manufactured, each opening is formedas a dosed cup, integral with the rest of the tank. This provides aseamless, airtight seal for any openings which are unused. When anopening is to be used, the inner face is cut out, preferably by cuttingparallel to, and flush with, the inner surface of the opening, leaving acircular cuff projecting into the interior of the tank. In a similarmanner, the alignment cups, 122, can be utilized for smaller capacityconnections. Various patterns of openings and connections can besupported by changing the end plate pattern in the mold. In thepreferred embodiment, all four end plates are identical, providingconnections which are symmetrical about all three major axes of thetank. If desired, any combination can be achieved including having eachplate different. The symmetry provided by the preferred embodiment hasbeen found desirable where multiple tanks are to be joined together intoa larger system.

FIG. 7 provides a detailed view of the coupling between two tanks, 124and 126, which are positioned flush together. The tanks are positionedwith their openings, 120A and 120B, aligned. If not already done, theend faces of these connection cups are cut out to form through openings.A length of suitable pipe, 132, is inserted through the openings so thatit extends slightly into the interior of each tank. While a minimumlength must be provided to form a good seal with the sleeves,significant excess length is not a problem, simplifying theinstallation. While not required, a slight taper, 134, on the outer edgeof the ends eases installation of the sleeves. The sleeves, 130A and130B, are slipped over the ends of the pipe and positioned to overlapboth the cuff of the openings and the pipe. Band clamps, 136, are thenpositioned near each end of both sleeves and tightened to form awatertight seal between the sleeve and the pipe, or opening, asappropriate. With the sleeve positioned on the inside of the tank, waterpressure from the contained water serves to tighten the seal since thehydraulic pressure forces the sleeve into tighter contact with the cuffof the opening and the pipe. This is in contrast to the conventionalarrangement with the connection on the outside of the tank wherehydraulic pressure is pushing the flexible coupling outward, expandingit away from its contact surfaces.

With the joint formed in this manner, there is no need for an opening orgap between the ends of the adjacent tanks. The tank surfaces can be incontact right up to the edge of the opening, around the fullcircumference of the pipe.

In the preferred embodiment, the inside diameter of the opening, 120, isslightly larger than the outside diameter of the pipe, 132. This allowsfor slight angular movement of the pipe relative to the centerline ofthe opening. This also allows for a certain amount of relative movementbetween two adjacent tanks which are interconnected. Such movement mightoccur during installation, as the fill settles, or as the surroundingsoil shifts due to freeze-thaw cycles.

It should be noted that all work involved in sealing the connectiontakes place from within one or the other of the tanks. Once the tanksare properly installed and aligned, there is no need for access to theoutside of the tank. If desired, the tanks may be immediatelybackfilled, partially or fully, without delaying for the installation ofthe sealing sleeves. The sleeve and band clamps are protected from theoutside environment by the tank. While exposed to the contents of thetank, they are not exposed to ground water, fill material, or topossible damage by workers tools as the fill is positioned andcompacted. The internal positioning of the joint also simplifiesinspection and repair of the joint after the tank is put into use.Rather than excavating the tank, it is merely drained to a level belowthe joint in question allowing the clamps and/or sleeves to be inspectedor replaced.

Minimal skill is required to properly install the components of theconnection. The pipe must be cut to length and inserted; the sleevesslipped into place; and the band clamps positioned and tightened. Thesesteps can be easily performed by unskilled labor. This contrasts with aflanged connection between metal, or possible concrete, tanks whichrequire a great deal of skill and could require the services of apipefitter. Where the installation is in a remote rural area, skilledlabor may not be readily available. The ability to install the tankswith unskilled labor greatly decreases the cost of a project and maydecrease the time required.

The materials used in making the connection are also common, likely tobe available in a rural area. While the sleeve is somewhat special, theembodiment illustrated in FIG. 7 is a commercially available part. Thepipe and band clamps are used for a variety of purposes and are widelyavailable. This availability of parts means that if a part is misplacedor damaged, it may be possible to replace it at the nearest agriculturalimplement dealer or building supply center, rather than shipping in apart from a large city or even the manufacturer. This may mean thedifference between finishing that day or finishing 3 to 5 days later.

The method of interconnecting the tanks also enables a method of repairwhich is not typically available. If one of the interconnecting pipesdevelops a crack or is damaged, as might occur if the soil freezes orwater in the pipe freezes, it can be repaired without excavating thetanks. From within the affected tanks, the sleeves, 130 A & B. areremoved and the pipe, 132, trimmed approximately flush with the end ofthe internal tank cuff, 120 A & B. A pipe of slightly smaller diameteris then inserted through the original pipe, extending beyond theopenings as before. A new sleeve, with a smaller minor end, is thenfitted which connects the cuff to the newly inserted pipe. While theflow capacity of the connection is somewhat reduced, the entire repairis performed without excavation and in the matter of minutes after thetanks are drained.

FIG. 7 also illustrates the functioning of the alignment cups, 122 A &B. These cups are sized to accept a short length of standard size PVCpipe, 128. In the preferred embodiment, 3 inch diameter pipe is used. Apipe section of the appropriate size is cut for each opposing pair ofalignment caps with length which is slightly less than twice the depthof an individual alignment cup. This allows the pipe to reach fully fromend to end of two adjacent cups, with a slight gap to prevent binding.As the second of two adjacent tanks is being installed, the pipesections are inserted into the cups of one of the tanks. As the secondtank is brought into alignment with the first tank, the pipe sectionswill slip into the alignment cups on the second tank, fixing the twotanks in position relative to each other. Because the pipe is not afight fit into the cup, a certain amount of relative movement can occurto accommodate minor misalignment of the tanks. This looseness alsoaccommodates tank-to-tank variations inherent in the manufacturingprocess. The tightness of this fit can be adjusted during manufacture toregulate the amount of movement allowed. In the preferred embodiment,the diameter of the cups is approximately ⅛ th inch larger than the pipefor a 3 inch pipe.

Alternatively, where spreader tubes are not used, the alignment cups mayalso be used for low volume fluid connections between the tanks, asmight be needed for septic tanks. The cups are made in the same manneras the large fluid connection cup, 120, and a connection can be made inthe same manner using smaller diameter pipe, sleeves and clamps.Similarly, the large fluid connection, 120, can also be used foralignment. A short section of the appropriate diameter pipe can beinserted into the large openings in the same manner as the alignmentcups are used. The openings may be left closed and cut open afterinstallation and the pipe section removed, or they may be cut open and alonger length of pipe, even the one to be used for the fluid connection,can be used for alignment.

FIGS. 8, 9, and 10 illustrate alternative embodiments of the elastomericsealing sleeve which can be used. Other forms are also anticipated. FIG.8 shows the simplest form of the sleeve, 130, in which the sleevetransitions from one diameter to the other. This sleeve provides a goodseal, but only limited movement of the pipe. This amount of movement isprobably sufficient for most installations and this embodiment has theadvantage of being commercially available. FIG. 9 illustrates a humpedform of the sleeve, 140. The hump increases the flexibility of thesleeve, increasing both the angular and longitudinal range of movementwhich can be tolerated. This form may have to be custom manufactured.FIG. 10 illustrates the embodiment of the sleeve, 142, which has thegreatest range of movement. The S-curve can tolerate a significantamount of longitudinal movement as well as angular movement. Typically,this form would be combined with a somewhat smaller diameter pipe, 138,both to accommodate the S-curve and to take advantage of it. By reducingthe diameter of the pipe, the amount of angular deflection until thepipe contacts the circumference of the opening is substantiallyincreased. A ratio of 75% to 90% between the pipe outer diameter and thecuff inner diameter have been found to provide good performance. Angulardeflections of 5 to 10 degrees between the pipe and the centerline ofthe opening can be easily accommodated by this design. (In contrast tothe use of the flared boot and normal size pipe which provides forapproximately 3 degree deflections.) This is the best combination wheresignificant movement is anticipated. This form of sleeve may also haveto be custom manufactured. A further advantage of this design over thehumped design of FIG. 9 is that it is easier to manufacture usingstandard injection molding tooling. This form can be generated using asimple internal plug to form the inside of the sleeve, whereas thehumped version would require a contracting plug or a two-piece,separable outer die, which increases both cost and cycle time.

FIG. 11 illustrates one of many possible configuration options forcombining several tanks as modules in a larger system. In the preferredembodiment, each tank holds approximately 2000 gallons. A system ofalmost any size can be built, in nominal increments of 2000 gallons byinterconnecting tanks. In the illustration, tanks 144 and 146 have beeninstalled with their adjoining faces flush. This is the preferredinstallation method for most cases since it minimizes the pipe length(maximizing the flow, and minimizing the cost) and minimizes the size ofexcavation required. Tanks 146 and 148 have been installed with anintervening gap and a longer interconnect pipe, 150. This simplealternative increases the installation options and can reduce cost. Asan example, if a rock outcropping intrudes into the excavation, it maybe possible to offset the tanks to either side of the outcropping andavoid the necessity of attempting to break out the rock. Where a singlelarge tank is used, this option is unavailable.

Further alignment options are illustrated in FIG. 12. There is norequirement that the connections between two tanks be straight. Tanks152 and 154 are illustrated as being installed at an angle, using eitherconventional or custom pipe couplings. This would be desirable to avoida rock outcropping, as above, or to allow the excavation to follow thecontour of a hillside. More radical relative angles are also possible asbetween tank 154 and 156. It is also possible to configure tanks insquare or rectangular arrangements with two or more parallel rows. Thetanks may then be interconnected as a single sequential system, wherethe pipes snake between the rows; as parallel subsystems which thenmerge into a single output; or as multiple separate systems placed in asingle excavation. Other configurations are clearly possible, such ascascaded subsystems.

The preferred material for the interconnect pipe is common PVC. It isrelatively rigid, easily worked, and widely available. However almostany form of tubing or semi-rigid hose can be used. The use of flexiblehose provides for a continuous range of angles between the tanks,greater vertical or horizontal displacement between the tanks and willtolerate significant relative movement between the tanks afterinstallation. Note that where the pipe length required is greater thanthe interior length of the tank, the pipe will have to be inserted intoat least one of the tanks prior to placing the next tank. Where thedistance is less than this, both tanks may be placed and the pipe passedinto one tank through the access hatch.

In a typical installation, the tank exhibits good strength andstructural rigidity. The modified sphere resists uniform external loadsas would be exerted by the surrounding fill. This is assisted by theinternal hydraulic pressure of the stored water. However, if the tank isleft empty for extended periods, while the surrounding soil is shifting,the tank may deform slightly. An example of such a situation is where apotable water storage tank is drained over the winter, either to avoidfreezing or stagnation, and the soil experiences multiple freeze-thawcycles. The portions of the tank most prone to deformation are the flatends, where the end does not abut an immediately adjacent tank. Analternative embodiment of the tank, shown in FIG. 13, addresses thisproblem. Spreader tubes, 168, are installed to hold the ends uniformlyapart. The preferred embodiment is to use lengths of PVC pipe of thecorrect diameter to fit snugly over the internal profile of thealignment cups . Note that this diameter will be larger than for thepipe sections used for external alignment. The pipes are preferably cutto the same length as the interior of the tank providing a snug fit withthe spreaders captured by the alignment cups. Alternatively, thespreaders could be cut slightly longer than the interior of the tank,resulting in the end being slightly bowed. The spreaders are installedas shown in FIG. 14. One end of the pipe, 168, is placed over analignment cup and the opposite end is positioned near the correspondingcup at the other end. A ramp plate, 170, forms an incline from the wallof the tank to the top of the cup, 122. The pipe is then forced towardthe cup, riding up the ramp, and dropping over the alignment cup. Whilethis process is preferably performed at the factory, it can also be donein the field.

The features of the present invention, as described above and asillustrated, combine to offer a modular tank design with severaladvantages over others presently available. Installation andinterconnection of the tank is very simple. The individual tanks aresmall and lightweight, making them easy to handle, transport, andposition. Large equipment is not needed. The tanks are easily handled bya back-hoe or skid steer loader. If necessary they can be maneuveredwith manual labor. The provision of the alignment cups, and the similarutility of the fluid connection, simplifies relative alignment ofadjacent tanks. The flexible coupling is adaptable to varying degrees ofmisalignment between the tanks. If necessary or preferred, the tanks maybe spaced apart and connected by a longer section of pipe, allowing theinstallation to adapt to the environment. The connection itself is madewith common materials and requires little skill to perform correctly.Being inside the tank, the connection is aided by the hydraulic pressureof the contained fluid rather than working against it. It is alsoprotected from environmental factors such as ground water, abrasive fillmaterial, and damage from shovels and other tools during installation.After installation, the connections can be inspected and repaired fromwithin the tank, without requiring excavation.

While the design is not restricted to a single material, rotary moldedpolyethylene has been found to provide the best performance in thepreferred embodiment. This material is inexpensive to form, is impacttolerant and corrosion resistant. The rotary molding process results ina seamless, one piece tank. This is especially beneficial for potablewater storage where the lack of seams reduces the likelihood of groundwater infiltration and the smooth interior resulting from the rotarymolding process eliminates seams, fissures, or joints which could harborbacteria. The use of other materials would allow this design to bereadily adapted to other applications such as chemical or fuel storage.

The modular approach provided by the present invention lends itself toinstallation of storage systems in remote or restricted accesslocations. The small size of the individual tanks makes them easy totransport. A single tank can be carried in a pickup truck or on a smalltrailer making access via a single lane dirt road viable. The lightweight means that a tank can be unloaded and placed with relativelysmall equipment, such as a skid steer loader or farm tractor, ratherthan requiring a crane. The adaptability to various configurations meansthat the installation can be matched to the terrain, significantlyreducing excavation costs and impact on the surrounding area. Anessentially unlimited number of tanks can be combined to create a systemof any desired capacity. In the preferred embodiment, only a single sizeof tank is used, but a combination of sizes could be provided,increasing the total capacity options for the system. Systems of thetanks according to the present invention can be used for potable waterstorage, fire protection, and septic systems as well as otherapplications.

While the preferred form of the invention has been disclosed above,alternative methods of practicing the invention are readily apparent tothe skilled practitioner. The above description of the preferredembodiment is intended to be illustrative only and not to limit thescope of the invention.

I claim:
 1. A fluid containment tank comprising: (a) a cylindrical mainbody, having a longitudinal axis, comprising plural alternating outwardand inward ribs, each of said outward ribs being substantially circularand lying in a plane substantially normal to said longitudinal axis andeach of said inward ribs comprising plural interconnected straightsections, of substantially equal length, forming a closed polygon, andall of said sections in a plane substantially normal to saidlongitudinal axis; (b) two arcuate end caps integrally formed with saidmain body thereby forming a substantially closed, fluid tight container;and (c) at least one fluid tight, integrally formed fitting adapted tosubsequent modification for use as a fluid connection.
 2. The fluidcontainment tank of claim 1 wherein said fitting extends toward theinterior of said tank.
 3. The fluid containment tank of claim 2 furthercomprising at least one integrally molded alignment cup extending intothe interior of said tank from a first of said end caps.